Spiral wound gas separation membrane module

ABSTRACT

A gas separation module comprising one or more gas separation elements, said elements comprising at least two membrane sheets and a permeate carrier sandwiched between the membrane sheets, wherein the contact area of the membrane sheets with the permeate carrier is less than 50%.

RELATED APPLICATION DATA

This application is a National Stage Application under 35 U.S.C. 371 ofco-pending PCT application number PCT/GB2014/052942 designating theUnited States and filed Sep. 30, 2014; which claims the benefit of GBapplication number 1317519.5 and filed Oct. 3, 2013 each of which arehereby incorporated by reference in their entireties.

This invention relates to gas separation modules and to their use.

Gas separation modules typically comprise one or more gas separationelements, wherein each element comprises at least two membrane sheetsand a permeate carrier sandwiched between said membrane sheets. Thepermeate carrier is usually a smooth, gas permeable material whichcontacts the membranes across all or most of their surfaces, such thatthe contact area of the membrane sheets with the permeate carrier isvery high (in many cases approaching 100%). The membrane sheetstypically comprise a polymeric discriminating layer and a poroussupport.

Often the gas separation elements are separated from each other by feedspacer screens, which are of a relatively large mesh size to accommodatefeed gas flow. Typically a stack of alternating gas separation elementsand feed spacers are wound spirally onto a perforated permeatecollection tube which collects gas which has permeated into the gasseparation element.

In spiral gas separation modules, the outside edges of the gasseparation elements are generally sealed on all but one side, allowingaccess to the permeate carrier only from a radial direction through themembrane. The gas separation elements are placed with the unsealed edgeadjacent to a perforated permeate collection tube and oriented along thelength of the tube, allowing the permeate to flow into the permeatecollection tube.

After the gas separation elements are wound onto a permeate collectiontube, some type of external restraining means such as a hard shell,straps, anti-telescoping device or a bypass screen, or a combinationthereof, may be used to hold the spiral wound gas separation elements intight formation around the tube. The spiral module is then loaded into apipe-like housing or pressure vessel which is operated at a slightpressure drop across the module as the gas being filtered flows through.

The present inventors have found that the modules having good flux andselectivity may be obtained by reducing the contact area of the membranesheets with the permeate carrier as described below.

According to a first aspect of the present invention there is provided agas separation module comprising one or more gas separation elements,said elements comprising at least two membrane sheets and a permeatecarrier sandwiched between the membrane sheets, wherein the contact areaof the membrane sheets with the permeate carrier is less than 50%.

The term “comprising” is to be interpreted as specifying the presence ofthe stated parts, steps or components, but does not exclude the presenceof one or more additional parts, steps or components.

Reference to an item by the indefinite article “a” or “an” does notexclude the possibility that more than one of the item(s) is present,unless the context clearly requires that there be one and only one ofthe items. The indefinite article “a” or “an” thus usually means “atleast one”.

The contact area of said membrane sheets with the permeate carrier ispreferably <45%. The contact area of said membrane sheets with thepermeate carrier is preferably >5%, more preferably >10%,especially >20%. The contact area of each membrane sheet with thepermeate carrier may be determined by forming a membrane sheet—permeatecarrier laminate using a laminator, peeling off the membrane sheets anddetermining the contact area by performing measurements using amicroscope. The contact area can be seen as an impression on themembrane sheet (see FIG. 1 and FIG. 2. and the discussion the discussionof these figures below). A suitable laminator is the Ibico PL-330LSI.Typically the laminator operates at 80° C. A suitable microscope forperforming the contact surface area measurements is the Nikon SZM800,using CellSens Dimension 1.6 digital imaging software to analyse imagesobtained using the microscope. In some cases, e.g. when the membrane istemperature-sensitive, it is convenient to place the permeate carrierinside a lamination pouch made from a mixture of a polyester and EVA(ethylene vinyl acetate) (instead of placing the permeate carrierbetween two membranes) and to obtain the contact area using that pouchinstead.

Preferably at least 50%, more preferably at least 75%, especially 100%of the gas separation elements present in the module comprise at leasttwo membrane sheets and a permeate carrier sandwiched between themembrane sheets, wherein the contact area of the membrane sheets withthe permeate carrier is as stated above (e.g. less than 50%).

In one embodiment, the (cross sectional) area of the membrane sheet andpermeate carrier are identical, for example the length and width of themembrane sheet and permeate carrier are identical. In anotherembodiment, the area of the membrane sheet and permeate carrier are notidentical, for example when the length and/or width of the permeatecarrier is smaller than length and/or width of the membrane sheets. Whenthe area of the membrane sheet and permeate carrier are not identical,the contact area is calculated relative to the area of the permeatecarrier. For example, the area of rectangular permeate carrier is itslength×width and the % contact area is calculated relative to the areaof the permeate carrier (length×width).

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a photograph showing a contact area of less than 50%.

FIG. 2 is a photograph showing a contact area of more than 50%.

FIG. 3 is a partially exploded, perspective view of a gas separationmodule of the present invention.

FIG. 4 illustrates a device used to measure the axial flux rate of apermeate carrier.

FIG. 1 is a photograph of a polyester/EVA sheet which has been laminatedin contact with the course side of macroporous sheet HW 2503 which canbe used as a permeate carrier, as mentioned in Table 5 below. One canclearly see the contact areas (2) as the impression embedded into thepolyester/EVA sheet. The smooth, non-contact areas (1) can also be seen.In this case the contact area was 42.4%.

FIG. 2 is a photograph of a polyester/EVA sheet which has been laminatedin contact with the wales side of macroporous sheet HW 2503. One canclearly see the contact areas (2) as the impression embedded into thepolyester/EVA sheet. The smooth, non-contact area (1) can also be seen.In addition, there was a fine diamond-pattern (4) arising from nettingbetween the polyester/EVA sheet and the permeate carrier which furtherreduced the contact area slightly. In this case the contact area was54.2% (i.e. outside of claim 1).

Referring to FIG. 3, a gas separation module according to the presentinvention is designated generally by the numeral (10). The module has acentral permeate collection tube (16) having perforations (22) along itslength. The module includes gas separation elements (12) wound aroundthe permeate collection tube. The gas separation element (12) includesmembrane sheets (18) and a permeate carrier (20) sandwiched between themembrane sheets (18). The contact area between the membrane sheets (18)and a permeate carrier (20) is selecting one or the other to have asurface profile which reduces the contact area to below 50% (e.g. byusing a highly textured permeate carrier (20)) (not shown). Each gasseparation element is oriented to present an edge generally adjacent thetube (16), a pair of side edges and an axial edge distal from the tubeand oriented to be in parallel with the axis of the tube. A liquidadhesive (not shown) is applied between the membrane sheets (18) alongthree sides and in a location near the axial and side edges of theelement that corresponds to the desired location for a gas-tight seam,i.e. the periphery. The fourth, open edge of the gas separation element(12) is fixed onto the permeate collection tube (16) so that gas whichhas permeated into the gas separation element can flow throughperforations (22) and into the permeate collection tube (16). The edgesbetween adjacent gas separation elements (12) which lie along the axiallength of permeate collection tube (16) are sealed or folded on the edgewhich contacts the permeate collection tube (16) so that feed gas canflow through the optional feed spacer screens (14) between the gasseparation elements but cannot enter the permeate collection tube (16)without first passing through the membrane sheets (18) and into thepermeate carrier (20).

Permeate carrier (20), membrane sheets (18), and feed spacer screens(14) are thus spirally wound around permeate collection tube (16) withpermeate carrier (20) located adjacent to tube (16) and in gascommunication therewith. Typically a plurality of gas separationelements (12), each comprising at least two membrane sheets (18) and apermeate carrier (20) sandwiched between the membrane sheets (20) arespirally wound about permeate tube (16) with a feed spacer screen (14)located between each gas separation element (12). The module mayoptionally be formed without feed spacer screen (14).

The permeate carrier preferably has an average thickness of 150 to 800μm, preferably 200 to 500 μm, especially 250 to 400 μm.

The desired contact area may be achieved by using membrane sheets and/orpermeate carriers having a surface profile which causes the desiredcontact area. For example, one may use highly textured membrane sheetsor, more typically, highly textured permeate carriers whereby thetexture results in a low contact area between the membranes and thepermeate carrier. The surface profile(s) can be used to keep >50% of thesurface of the facing membrane sheets and permeate carriers apart, whileat the same time ensuring there is some contact between the membranesheets and permeate carriers, albeit a contact area <50%.

The permeate carrier preferably comprises a macroporous sheet (e.g. asheet having pores of average size >30 μm. The macroporous sheetstypically have very high gas permeability. The macroporous sheets arenot included to discriminate between gases but instead to provide apathway for the permeate gases to flow through. Suitable macroporoussheets include woven fabric, non-woven fabric, especially knittedfabric, more especially a warp knitted fabric or a weft knitted fabric.Knitted fabrics typically comprise a plurality of consecutive rows ofloops called ‘stitches’. As each row progresses, a new loop is pulledthrough an existing loop. The active stitches are held on a needle untilanother loop can be passed through them. This process eventually resultsin a knitted fabric. Knitting may be done by hand or more typically bymachine.

Suitable weft knitted fabrics can be made from one yarn, although morethan one yarn can be used to achieve particular patterns and surfaceprofiles in the fabric and create a surface texture which gives rise tothe desired contact area. The yarn is typically inserted in a horizontalor weft direction, hence the classification as weft knitted.

Rows of stitches in knitted fabrics are called ‘courses’ and columns ofstitches are called ‘wales’.

Warp knitted fabrics are also composed of loops arranged in wales andcourses. The yarn is typically inserted in the vertical or warpdirection, hence the classification as warp knitted. They require atleast one warp yarn to supply each needle on a knitting machinemanufacturing the warp knitted fabric. They are normally made with 2 ormore sets of warp yarns. Their properties normally lie between those ofwoven and weft knitted fabrics.

A particularly preferred warp knitted fabric is tricot. In tricotfabrics the yarn typically zigzags vertically along columns of knitresulting in a series of wales (ribs) on one side and on the other(back) side is the course side where the courses are in series parallelto the orientation of the wales. The orientation in the presentinvention as such is not limited as long as the side facing the membranehas a contact area of less than 50%.

The permeate carrier optionally has a different surface profile on eachside. For example, one side may comprise mostly ‘courses’ (referred toas the course side) and the other side may comprise mostly ‘wales’(referred to as the wales side). When the permeate carrier has one sidewhich would result in a contact area of less than 50% (e.g. the courseside) and another side which would result in a contact area of 50% ormore (e.g. the wales side), one will orientate the permeate carrier suchthat the side which would result in a contact area of less than 50% isin contact with the membrane (e.g. the course side).

The permeate carrier optionally comprises two or more macroporous sheets(e.g. fabrics), which may be the same or different. For example, whereone side of the macroporous sheet is rough and the other relativelysmooth, one may orientate two sheets of the macroporous sheet ‘back toback’ with the relatively smooth faces in contact with each other suchthat the relatively rough faces contact the membranes and achieve thedesired contact area. As macroporous sheet one may also use, forexample, the materials mentioned below as being suitable for making theporous support for the composite membrane.

One may choose macroporous sheets having an uneven surface profile inorder to ensure that the desired contact area between the permeatecarrier and the membrane(s) is kept below 50%.

The permeate carrier is optionally made from a natural fibre or aman-made fibre, e.g. polyester, polysulfone, polyester, nylon, teflon,polypropylene, polyphenylenesulfide, etc. The fibres are optionallyresin coated, e.g. with a resin such as an epoxy or melamine resin.

The permeate carrier preferably comprises at least two macroporouslayers and a gas impermeable sheet (sometimes called an interfoil)located between the two macroporous layers. The presence of such a gasimpermeable sheet can improve selectivity, gas flux and also reducecompaction of the gas separation element arising from the high pressuresexperienced by the module. Suitable gas impermeable sheets preferablyhave a thickness of <0.1 cm, more preferably <0.07 cm.

The macroporous layers may be provided by one or more macroporoussheets. In one embodiment the macroporous layers are provided by two ormore macroporous sheets. For example, the permeate carrier comprises atleast two macroporous sheets and a gas impermeable sheet located betweenthe macroporous sheets, preferably being coextensive therewith. In analternative embodiment, the macroporous layers are provided by onemacroporous sheet folded around the gas impermeable sheet. For example,the permeate carrier comprises one macroporous sheet and a gasimpermeable sheet of half the cross sectional area of the macroporoussheet and the macroporous sheet is folded around the gas impermeablesheet, preferably such that the macroporous sheet and the gasimpermeable sheet are substantially coextensive.

Preferably the gas separation module is a spiral wound gas separationmodule. For example, the gas separation further comprises a perforatedpermeate collection tube and the one or more gas separation elements arewound around that tube and are in gas communication therewith.

The function of the permeate collection tube, when present, is tocollect the permeate gas which has passed through the membranes. Thusthe elements are arranged such that the permeate can flow through thepermeate collection tube perforations and the retentate cannot flowthrough the permeate collection tube perforations.

The openings along the length of the permeate collection tube allow gasflow from the exterior of tube to the interior. Surrounding the permeatetube and in gas communication therewith is a permeate carrier. Thepermeate carrier typically transports the filtered permeate in adirection perpendicular to the axial length of the tube.

The permeate collection tube is typically constructed of a rigidmaterial, for example a metal (e.g. stainless steel) or a plastic.

Typically the membrane sheets are composite membranes, e.g. comprising adiscriminating layer and a porous support. The function of thediscriminating layer is to preferentially discriminate between gases,separating a feed gas mixture into a permeate which passes through themembrane and a retentate which does not pass through the membrane. Thepermeate and retentate typically comprise the same gases as the feed gasmixture, but one is enriched in at least one of the gases present in thefeed gas and the other is depleted in that same gas.

The porous support is typically open pored, relative to thediscriminating layer. The porous support may be, for example, amicroporous organic or inorganic membrane, or a woven or non-wovenfabric. The porous support may be constructed from any suitablematerial. Examples of such materials include polysulfones,polyethersulfones, polyimides, polyetherimides, polyamides,polyamideimides, polyacrylonitrile, polycarbonates, polyesters,polyacrylates, cellulose acetate, polyethylene, polypropylene,polyvinylidenefluoride, polytetrafluoroethylene, poly(4-methyl1-pentene) and especially polyacrylonitrile.

One may use, for example, a commercially available, porous sheetmaterial as the support for the composite membrane. Alternatively onemay prepare the porous support using techniques generally known in theart for the preparation of microporous materials. In one embodiment onemay prepare a porous, non-discriminatory support by curing curablecomponents, then applying further curable components to the formedporous support and curing such components thereby forming the layer ofcured polymer and the discriminating layer on the already cured poroussupport. One may also use a porous support which has been subjected to acorona discharge treatment, glow discharge treatment, flame treatment,ultraviolet light irradiation treatment or the like, e.g. for thepurpose of improving its wettability and/or adhesiveness.

The porous support preferably used to form the membrane preferably hasan average pore size of at least about 50% greater than the average poresize of the discriminating layer, more preferably at least about 100%greater, especially at least about 200% greater, particularly at leastabout 1000% greater than the average pore size of the discriminatinglayer.

The pores passing through the porous support typically have an averagediameter of 0.001 to 10 μm, preferably 0.01 to 1 μm (i.e. before theporous support has been converted into a composite membrane). The poresat the surface of the porous support will typically have a diameter of0.001 to 0.1 μm, preferably 0.005 to 0.05 μm. The pore diameter may bedetermined by, for example, viewing the surface of the porous support byscanning electron microscopy (“SEM”) or by cutting through the supportand measuring the diameter of the pores within the porous support, againby SEM. The porous support preferably has an average thickness of 20 to500 μm, preferably 50 to 400 μm, especially 100 to 300 μm.

One may use an ultrafiltration membrane as the porous support, e.g. apolysulfone ultrafiltration membrane, cellulosic ultrafiltrationmembrane, polytetrafluoroethylene ultrafiltration membrane,polyvinylidenefluoride ultrafiltration membrane and especiallypolyacrylonitrile ultrafiltration membrane. Asymmetric ultrafiltrationmembranes may be used, including those comprising a porous polymermembrane (preferably of thickness 10 to 150 μm, more preferably 20 to100 μm) and optionally a woven or non-woven fabric support. The poroussupport is preferably as thin as possible, provided it retains thedesired structural strength.

Typically the discriminating layer is present on one side of the poroussupport or is partially or wholly within the porous support.

Preferred discriminating layers comprise a polyimide, especially apolyimide having —CF₃ groups. Polyimides comprising —CF₃ groups may beprepared by, for example, the general methods described in U.S. Pat.Reissue No. 30,351 (based on U.S. Pat. No. 3,899,309) U.S. Pat. No.4,717,394 and U.S. Pat. No. 5,085,676. Typically one or more aromaticdianhydrides, preferably having —CF₃ groups, are condensed with one ormore diamines. The diamine(s) and dianhydride(s) copolymerise to form anAB-type copolymer having alternating groups derived from the diamine(s)and dianhydride(s) respectively.

Preferably the discriminating layer comprises groups of the Formula (1)wherein Ar is an aromatic group and R is a carboxylic acid group, asulphonic acid group, a hydroxyl group, a thiol group, an epoxy group oran oxetane group:

Optionally the membranes further comprise a polymeric layer between theporous support and the discriminating layer, often referred to as agutter layer. Preferred gutter layers comprise a dialkylsiloxane.

Preferably the module further comprises a feed spacer screen, locatedbetween the separation elements. The feed spacer screen typically has arelatively large mesh size to allow the feed gas to travel axially alongmembrane module (in the case of a spiral gas module). In most instances,the feed spacer screen will be utilized, but it is possible to constructa module without this component. In general, a feed spacer screen isformed of any inert material which maintains a space between the gasseparation elements and is stable to the feed gas. Further, the feedspacer screen allows the gas to be filtered to travel axially along themembrane module.

Preferred materials for the feed spacer screen are open, channel forminggrid materials, such as polymeric grid, or corrugated or mesh materials.Preferred among these are polypropylene or other polyolefin nettingmaterials.

Typically the edges of adjacent membrane sheets which lie along theaxial length of permeate tube are sealed so that gas flowing throughfeed spacer screen is prevented from direct access to permeate tube.Alternatively, the membrane sheet may be folded with the fold beingadjacent to the permeate tube and with feed spacer screen located withinthe fold such that membrane surfaces face one another.

The permeate carrier, the membrane sheets, and feed spacer screen(s)(when present) may thus be spiral wound around a permeate collectiontube with the permeate carrier in gas communication with the permeatecollection tube. Referring to the series of layers of membrane sheet,permeate carrier and a second membrane sheet as a gas separationelement, typically a plurality of gas separation elements are spiralwound about the permeate tube with a feed spacer screen located betweeneach element.

Preferably at least one of the gas separation elements reduces inthickness by <25%, more preferably <23%, when it is subjected to apressure of 7 million Pascal for 5 hours.

After the membrane module has been wound, the assembly may be held in awound state through the use of restraining bands or outer wraps, or acombination thereof. A preferred method of restraining the assembly isby filament winding, in which a glass fibre filament dipped in anadhesive is wound around the assembly and cured. The modules can then beloaded into a housing or pressure vessel which is preferably operated ata slight pressure drop across the module as the gas being filtered flowsthrough. In operation, the feed gas to be filtered is introduced at oneend face of the membrane module.

In a spiral wound gas separation module, the feed gas travels axiallyalong membrane module through the feed spacer screen. As the feed gasencounters the external surface of a gas separation element, part of thefeed gas (the permeate) passes through membrane in a directionperpendicular to the axis of tube. After the permeate passes through themembrane, it travels along the permeate carrier, eventually passing intopermeate tube through the perforations. The permeate exits the membranemodule through the permeate tube and the retentate travels axiallythrough the module along feed spacer screen.

As will be appreciated, in a spiral wound gas separation module it isnecessary to seal all of the edges of membrane sheets, with theexception of the edge adjacent to the permeate tube, in order to preventthe feed gas from entering the permeate carrier without first passingthrough the membrane. Thus it is necessary to prevent the feed gas fromentering permeate carrier without first being filtered as desired. Inthe method of preparing the module according to the present invention,an adhesive may be applied to at least a part of the periphery of themembrane sheets, e.g. to the side edges and axial edges

Preferably the membranes have a CO₂/CH₄ selectivity (αCO₂/CH₄)>10.Preferably the selectivity is determined by a process comprisingexposing the membrane to a 13/87 mixture by volume of CO₂ and CH₄ at afeed pressure of 6000 kPa at 40° C.

While this specification emphasises the usefulness of the modulesprepared by the method of the present invention for separating gases(which includes vapours), especially polar and non-polar gases, it willbe understood that the modules can also be used for other purposes, forexample providing a reducing gas for the direct reduction of iron ore inthe steel production industry, dehydration of organic solvents (e.g.ethanol dehydration), pervaporation, oxygen enrichment, solventresistant nanofiltration and vapour separation.

According to a second aspect of the present invention there is provideda process for separating and/or purifying a feed gas comprising at leasttwo different gaseous components comprising passing the feed gas througha module according to the first aspect of the present invention suchthat feed gas is separated into a permeate gas and a retentate gas, oneof which is enriched in at least one of the said gaseous components andone of which is depleted in at least one of the said gaseous components.

The modules prepared according to the invention are particularly usefulfor the separation of a feed gas (which includes a feed vapour)containing a target gas into a gas stream rich in the target gas and agas stream depleted in the target gas. For example, a feed gascomprising polar and non-polar gases may be separated into a gas streamrich in polar gases and a gas stream depleted in polar gases. In manycases the membranes have a high permeability to polar gases, e.g. CO₂,H₂S, NH₃, SO_(x), and nitrogen oxides, especially NO_(x), relative tonon-polar gases, e.g. alkanes, H₂, N₂, and water vapour. The target gasmay be, for example, a gas which has value to the user of the module andwhich the user wishes to collect. Alternatively the target gas may be anundesirable gas, e.g. a pollutant or ‘greenhouse gas’, which the userwishes to separate from a gas stream in order to meet productspecification or to protect the environment.

The modules are particularly useful for purifying natural gas (a mixturewhich predominantly comprises methane) by removing polar gases (CO₂,H₂S); for purifying synthesis gas; and for removing CO₂ from hydrogenand from flue gases. Flue gases typically arise from fireplaces, ovens,furnaces, boilers, combustion engines and power plants. The compositionof flue gases depend on what is being burned, but usually they containmostly nitrogen (typically more than two-thirds) derived from air,carbon dioxide (CO₂) derived from combustion and water vapour as well asoxygen. Flue gases also contain a small percentage of pollutants such asparticulate matter, carbon monoxide, nitrogen oxides and sulphur oxides.Recently the separation and capture of CO₂ has attracted attention inrelation to environmental issues (global warming).

The modules of the invention are particularly useful for separating thefollowing: a feed gas comprising CO₂ and N₂ into a gas stream richer inCO₂ than the feed gas and a gas stream poorer in CO₂ than the feed gas;a feed gas comprising CO₂ and CH₄ into a gas stream richer in CO₂ thanthe feed gas and a gas stream poorer in CO₂ than the feed gas; a feedgas comprising CO₂ and H₂ into a gas stream richer in CO₂ than the feedgas and a gas stream poorer in CO₂ than the feed gas, a feed gascomprising H₂S and CH₄ into a gas stream richer in H₂S than the feed gasand a gas stream poorer in H₂S than the feed gas; and a feed gascomprising H₂S and H₂ into a gas stream richer in H₂S than the feed gasand a gas stream poorer in H₂S than the feed gas.

The invention will now be illustrated by the following, non-limitingexamples.

EXAMPLES

The contact area, axial flux rate, compaction gas flux and selectivitydescribed in the Examples were measured by the following techniques:

(A) Contact Area

In the Examples, the contact area was measured as follows:

A laminate was formed by sandwiching a square of the permeate carrierunder test (5 cm×5 cm) inside a pouch formed from two A6 sheets ofpolyester/ethylene vinyl acetate (EVA) (each sheet had a thickness of125 microns and the sheets were obtained from Staples) and feeding thepouch through a laminator (Ibico PL-330LSI) at 80° C. at motor speed 2.The polyester/EVA sheets were peeled off and contact areas of both thecourse (smooth) side and the wales (ribbed) side of the of the permeatecarrier were measured using a Nikon SZM800 microscope using CellSensDimension 1.6 digital imaging software version.

(B) Axial Flux Rate

The axial flux rate (l/min) of the permeate carriers was determinedusing the device illustrated in FIG. 4. In FIG. 4, one can see the 50bar of N₂ pressure being applied uniformly to the PE-interfoil whichthen uniformly compresses the membrane-permeate carrier laminate whichis glued on two sides. Air was applied to the right (open) side of thepermeate carrier at a pressure of 6 bar and the flow rate after 1 meterwas measured to give the axial flux rate (l/min). Flux rates of at least40 l/min are desirable under these conditions.

(C) Compaction

Compaction of the gas separation element was determined by measuring the% reduction in thickness of the element arising from compression for 5hours at a pressure of 70 bar. Compaction was therefore given by theequation:Compaction (%)=(T _(i) −T _(f))/T _(i)×100%wherein:

-   -   T_(i) is the initial thickness of the gas separation element;        and    -   T_(f) is the thickness of the gas separation element after        compression at 70 bar for 5 hours.

The thicknesses were measured using Zwick Z010 Materials TestingMachine. T_(f) was measured while the compression force of 70 millionPascal was still being applied to the gas separation element.

(D) Evaluation of Gas Flux & Selectivity

The gas permeability and selectivity of the modules was determined asfollows:

The flux of CH₄ and CO₂ through the module was measured at 40° C. andgas feed pressure of 6000 kPa using a pressure housing and a feed gascomposition of 13 v/v % CO₂ and 87 v/v % CH₄. Flow, pressure, and gascomposition of each feed gas, permeate gas, and retentate gas wasmeasured and flux and selectivity was calculated according formulationdescribed in “Calculation Methods for Multicomponent Gas Separation byPermeation” (Y. Shindo et al, Separation Science and Technology, Vol.20, Iss. 5-6, 1985) with “countercurrent flow” mode. For fluxcalculation a module surface area of 29 m² was used, and flux unit GPUis equal to 7.5×10⁻⁹ Nm³/m²·kPa·s.

The flux of O₂ and N₂ through the module was measured at 50° C. and gasfeed pressure of 6000 kPa using a pressure housing and using compressedair as a feed gas. Flux and selectivity was calculated in the samecalculation method as above.

Membranes

The following materials were used to prepare the Membranes 1 and 2:

-   PAN is a porous support polyacrylonitrile L10 ultrafiltration    membrane from GMT Membrantechnik GmbH, Germany.-   UV9300 is SilForce™ UV9300 from Momentive Performance Materials    Holdings. This is thermally curable copolymer comprising at least 3    epoxy groups and linear polydimethyl siloxane chains. Furthermore,    this copolymer cures rapidly when irradiated with UV light in the    presence of a photo-initiator.-   I0591 is 4-isopropyl-4′-methyldiphenyliodonium    tetrakis(pentafluorophenyl) borate (C₄₀H₁₈BF₂₀I) from TCI (a    photo-initiator which is free from mono-epoxy compounds).-   Ti(OiPr)₄ is titanium (IV) isopropoxide from Dorf Ketal Chemicals.-   n-Heptane is n-heptane from Brenntag Nederland BV.-   MEK is 2-butanone from Brenntag Nederland BV.-   CH is cyclohexanone from Brenntag Nederland BV.-   PI is    poly([({2,3,5,6-tetramethyl-1,4-phenylenediamine}-alt-{5,5′-[2,2,2-trifluoro-1-(trifluoromethyl)ethane-1,1-diyl]bis(isobenzofuran-1,3-dione)})-co-[{5-carboxylic-1,3-phenylenediamine}-alt-{5    trifluoro-1-(trifluoromethyl)ethane-1,1-diyl]bis(isobenzofuran-1,3-dione)}])    obtained from Fujifilm Corporation.-   CA is cellulose acetate L-70 from Daicel Chemical Industries Ltd.-   PE Interfoil is a 50 μm thick gas-Impermeable polyester sheet sold    under the name Mylar A50, obtained from Dupont.-   PET means polyethylene terephthalate.    Preparation of Membrane Sheets 1 and 2 (M1 and M2)    Stage a) Preparation of a Partially Cured Polymer 1 (“PCP Polymer    1”)

A solution of a PCP Polymer 1 was prepared by heating the componentsdescribed in Table 1 together for 105 hours at 95° C. The resultantsolution of PCP Polymer 1 had a viscosity of about 64,300 mPas whenmeasured at 25° C.

TABLE 1 Ingredients used to prepare PCP Polymer 1 Ingredient Amount (w/w%) UV9300 75 Ti(OiPr)₄ 1.5 n-Heptane 23.5Stage b) Preparation of Radiation Curable Composition 1 (“RCC1”)

Portions of the solution of PCP Polymer 1 obtained in stage a) abovewere cooled to 20° C., diluted with n-heptane and then filtered througha filter paper having an average pore size of 2.7 μm. The remainingingredients indicated in Table 2 below were added to make RCC1 to 5 asindicated in Table 2 below.

TABLE 2 RCC1 Inert solvent n-Heptane (w/w %) 84.9 MEK (w/w %) 1.6 PCPPolymer PCP Polymer 1 (w/w %) 13.3 Photo-initiator I0591 (w/w %) 0.2Stage c) Preparation of Compositions Used to Form a Discriminating Layer

Compositions DSL1 and DSL2 were prepared by mixing the components shownin Table 3 and filtering the mixtures through a filter paper having anaverage pore size of 2.7 μm.

TABLE 3 DSL1 DSL2 PI (w/w %) 2.00 0 CA (w/w %) 0 1.3 CH (w/w %) 6.0094.77 MEK (w/w %) 92.00 3.8 I0591 (w/w %) 0 0.13Stage d) Preparation of Membrane Sheets

Membrane sheets were prepared using the combinations ofradiation-curable composition and discriminating layers described inTable 4.

The radiation-curable composition RCC1 was applied to a porous PANsubstrate (step a)) at a speed of 10 m/min by a meniscus dip coating andirradiated. Irradiation (step b)) was performed using a Light HammerLH10 from Fusion UV Systems fitted with a D-bulb and irradiating with anintensity of 16.8 kW/m (70%). The resultant gutter layer had a drythickness of 300 nm. The discriminating layer was formed on the gutterlayer using the composition DSL1 as indicated in Table 4, using ameniscus type coating T 10 m/min coating speed. In Example 1, thediscriminating layer comprised PI. In Example 2, the discriminatinglayer comprised cellulose acetate. The resultant membrane sheets weredried and tested in isolation (i.e. in the absence of the permeatecarrier). The test results are shown in Table 4 below.

TABLE 4 Membrane Membrane Sheet 1 Sheet 2 Example (“M1”) (“M2”)Radiation-curable Composition RCC1 RCC1 Coating speed (m/min) 10 10Coating amount (ml/m²) 3 3 Dry layer thickness of gutter layer (nm) 300300 Discriminating layer composition DSL1 DSL2 Coating amount (ml/m²)8.4 8.4 Dry layer thickness of discriminating layer (nm) 120 120Permeate Carriers

The macroporous sheets used in the Examples to form the permeatecarriers were obtained from the suppliers indicated in Table 5 below:

TABLE 5 Macroporous Sheets Average Macroporous Abbre- thickness Sheetsviation Supplier Description (mm) wpi cpi HW 2503 MPS1 Hornwood A fabric0.3 46 44 Inc. made from PET and epoxy resin 75:25. GF 42369 MPS2Guilford A fabric 0.3 62 57 made from PET and epoxy resin 80:20. GF36168 MPS3 Guilford A fabric 0.3 45 46 made from PET and epoxy resin75:25. S 2866-2 MPS4 Seiren A fabric 0.23 45 46 made from PETWpi means wales per 2.54 cm (inch) and cpi means courses per 2.5 cm(inch).Preparation of Gas Separation Elements Ex1-Ex4 and CEx1-CEx3 (NoCentral, Gas Impermeable Sheet)

Gas separation elements were prepared as follows:

Permeate carriers comprising two macroporous sheets were prepared bygluing three of the four edges of the macroporous sheets together,leaving one edge free to feed permeate into a central tube (fittedlater). All four edges of both sides of the resultant permeate carrierswere then glued to the membrane sheets to give a gas separation elementcomprising a permeate carrier (comprising two macroporous sheets) andhaving membranes sheets attached as the outermost layers, with nocentral gas-impermeable sheet, as illustrated schematically below:

The contact area, axial flux rate and compaction of the resultant gasseparation elements were measured by the methods described above and theresults are also shown in Table 6 below.

Preparation of Gas Separation Element Ex5 (Includes a Central, GasImpermeable Sheet)

Gas separation element Ex5 was prepared in an analogous manner toEx1-Ex4 and CEx1-CEx3 except that a gas impermeable sheet (PE interfoil)was included between the macroporous sheets (see illustration below), asillustrated schematically below:

The contact area, axial flux rate and compaction of the resultant gasseparation elements were measured by the methods described above and theresults are also shown in Table 6:

TABLE 6 Gas Separation Elements Macroporous Gas Sheet Orien- imper-tation (i.e. Con- Axial meable which side tact Flux Exam- Gas separationsheet faces the Area Rate ple element Layers present? membrane) (%)(l/min) Ex1 M1/MPS1/MPS1/M1 No Course 42.4 61 Ex2 M2/MPS1/MPS1/M2 NoCourse 42.4 58 Ex3 M1/MPS3/MPS3/M1 No Course 43.0 47 Ex4 M1/MPS4/MPS4/M1No Course 48.0 41 Ex5 M1/MPS1/PE/MPS1/ Yes Course 42.4 65 M1 CEx1M1/MPS1/MPS1/M1 No Wales 54.2 39 CEx2 M1/MPS2/MPS2/M1 No Course 55.4 34CEx3 M1/MPS2/MPS2/M1 No Wales 71.8 28

As can be seen from Table 6, the axial flux rate was better when thecontact area of the membrane sheets with the permeate carrier was lessthan 50%.

Preparation of Modules

The gas separation elements from Ex 1 and Ex5 were wound onto aperforated tube of diameter 5 cm together with feed spacer screens toallow the influx of feed gases to the side of the membranes opposite tothe permeate carrier. The non-glued edges were fixed to the tube suchthat gas which permeates through the membrane sheets could flow throughthe permeate carrier and into the central tube but gas which did notpermeate through the membrane sheets could not enter the central tube.The resultant structure was encased in fibreglass and anti-telescopingdevices were glued to each corner cylinder side to give gas separationmodules. The modules were then cased in a steel housing and tested forgas flux and selectivity by the above described methods, providing theresults shown in Table 7 below:

TABLE 7 Gas Separation Modules containing the gas Seperation Elements(“GSE”s) Module Module αCO₂/CH₄ αO₂/N₂ Com- Gas impermeable separationseparation paction GSE sheet present? (sel/flux)* (sel/flux)* (%) Module1 Ex1 No 14/20 4.65/9.3  23 Module 2 Ex2 Yes 15/50 4.99/11.4 22 *selmeans selectivity (α) and flux is in GPU units

As can be seen from Table 7, inclusion of a gas impermeable sheet(Module 2) resulted in improved selectivity, and gas flux compared toModule 1 lacking the gas impermeable sheet.

The invention claimed is:
 1. A gas separation module comprising one ormore gas separation elements, said elements comprising at least twomembrane sheets and a permeate carrier sandwiched between the membranesheets, wherein the contact area of the membrane sheets with thepermeate carrier is less than 50%, wherein: (i) the permeate carriercomprises at least two macroporous layers and a gas impermeable sheetlocated between the two macroporous layers; and (ii) the contact area isas determined before use by forming a membrane sheet—permeate carrierlaminate using a laminator, peeling off the membrane sheets anddetermining the contact area by performing measurements using amicroscope.
 2. The gas separation module according to claim 1 whereinthe contact area is less than 45%.
 3. The gas separation moduleaccording to claim 1 being a spiral wound gas separation module.
 4. Thegas separation module according to claim 1 wherein the membrane sheetsand/or the permeate carrier have a surface profile which causes thecontact area of claim
 1. 5. The gas separation module according to claim1 wherein the membrane sheets and/or the permeate carrier have a surfaceprofile comprising surface projections.
 6. The gas separation moduleaccording to claim 5 wherein the surface projections comprise ribsand/or grooves.
 7. The gas separation module according to claim 1wherein the permeate carrier comprises a knitted fabric.
 8. The gasseparation module according to claim 1 wherein the gas impermeable sheethas a thickness of less than 0.1 cm.
 9. The gas separation moduleaccording to claim 1 wherein at least one of the gas separation elementsreduces in thickness by <25% when it is subjected to a pressure of 7million Pascal for 5 hours.
 10. The gas separation module according toclaim 1 which further comprises a central tube and the one or more gasseparation elements are wound around that tube and are in gascommunication therewith.
 11. The gas separation module according toclaim 1 wherein the gas separation elements are fixed to the centraltube such that gas which permeates through the membrane sheets can flowthrough the permeate carrier and into the central tube but gas whichdoes not permeate through the membrane sheets cannot enter the centraltube.
 12. The gas separation module according to claim 1 which comprisestwo or more gas separation elements and a feed spacer screen locatedbetween the gas separation elements.
 13. A process for separating gasesand/or purifying a feed gas comprising at least two different gaseouscomponents comprising passing the feed gas through a module according toclaim 1 such that feed gas is separated into a permeate gas and aretentate gas, one of which is enriched in at least one of the saidgaseous components and one of which is depleted in at least one of thesaid gaseous components.
 14. The gas separation module according toclaim 1 wherein the membrane sheets and/or the permeate carrier have asurface profile comprising surface projections which causes the contactarea of the membrane sheets with the permeate carrier to be than 45%.15. The gas separation module according to claim 14 being a spiral woundgas separation module.
 16. The gas separation module according to claim1 wherein (i) the permeate carrier comprises at least two macroporouslayers and a gas impermeable sheet located between the two macroporouslayers; (ii) the gas separation elements are fixed to a central tubesuch that gas which permeates through the membrane sheets can flowthrough the permeate carrier and into the central tube but gas whichdoes not permeate through the membrane sheets cannot enter the centraltube; and (iii) the membrane sheets and/or the permeate carrier have asurface profile comprising surface projections which cause the contactarea of the membrane sheets with the permeate carrier to be less than45%.
 17. The gas separation module according to claim 16 wherein the gasimpermeable sheet has a thickness of less than 0.1 cm.
 18. A process forseparating gases and/or purifying a feed gas comprising at least twodifferent gaseous components comprising passing the feed gas through amodule according to claim 16 such that feed gas is separated into apermeate gas and a retentate gas, one of which is enriched in at leastone of the said gaseous components and one of which is depleted in atleast one of the said gaseous components.